Systems and methods for minimizing static leakage of an integrated circuit

ABSTRACT

A leakage manager system for adequately minimizing static leakage of an integrated circuit is disclosed. The leakage manager system includes a generator configured to generate a control signal to be applied to a sleep transistor. A monitor is configured to determine whether to adjust the control signal to adequately minimize the static leakage. In some embodiments, the monitor includes an emulated sleep transistor. A regulator is configured to adjust the control signal depending on the determination.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No. 11/998,762titled “Systems and Methods for Minimizing Static Leakage of anIntegrated Circuit”, filed Nov. 30, 2007 now U.S. Pat. No. 7,642,836,which is a Divisional Application from U.S. application Ser. No.11/900,971, titled “Systems and Methods for Minimizing Static Leakage ofan Integrated Circuit,” filed Sep. 14, 2007 (now U.S. Pat. No.7,382,178), which is a Continuation-in-Part of U.S. application Ser. No.10/996,739, titled “Systems and Methods for Minimizing Static Leakage ofan Integrated Circuit,” filed Nov. 24, 2004 (now U.S. Pat. No.7,279,956), which claims the benefit of U.S. Provisional Application No.60/586,565 titled “Systems and Methods for I/O and Power IslandManagement and Leakage Control on Integrated Circuits,” filed Jul. 9,2004. This application is also related to U.S. patent application Ser.No. 10/840,893, titled “Managing Power on Integrated Circuits UsingPower Islands,” filed May 7, 2004 (now U.S. Pat. No. 7,051,306). All ofthe above-referenced applications are hereby incorporated by referencein their entirety.

BACKGROUND OF THE INVENTION

One design goal for integrated circuits is to reduce power consumption.Devices with batteries such as cell phones and laptops particularly needa reduction in power consumption in the integrated circuit to extend thelife of the battery. Additionally, a reduction in power consumptionprevents overheating and lowers the heat dissipation of the integratedcircuit, which in some cases eliminates or simplifies heat sinks and/orfans required to cool the integrated circuit. As well, the reduction inpower consumption of the integrated circuit reduces the AC power drawfor the device containing the integrated circuit.

A competing design goal for integrated circuits is increasedperformance. One way to increase performance is by increasing a maximumoperating frequency of a circuit. In order to increase the maximumoperating frequency of a circuit, or to integrate more functionality ina smaller area, integrated circuit manufacturing technology shrinks thedevice size of individual components (e.g. transistors) on theintegrated circuit.

However, as component device size scales from 250 nanometers to 130nanometers or below, a current draw of a device in standby mode referredto as static leakage becomes an increasingly large part of the powerbudget of the integrated circuit. For example, simulations show that,for an integrated circuit dissipating 50 watts constructed using 130nanometer devices, greater than 20 percent of the power dissipated isdue to static leakage. For even smaller devices, simulations show thatthe static leakage of an integrated circuit using 50 nanometer featuresizes comprises about 50 percent of the total power budget.

One solution for reducing static leakage includes use of one or moresleep transistors coupled to a logic gate of the integrated circuit.Application of a control signal to the sleep transistor may reduce thestatic leakage of the logic gate.

SUMMARY OF THE INVENTION

A system for minimizing static leakage of an integrated circuitcomprises a charge pump, an adaptive leakage controller, and a negativevoltage regulator. The charge pump generates a negative voltage to beapplied to a sleep transistor. The sleep transistor is configured tocontrol the static leakage of a logic gate of the integrated circuit. Insome embodiments, the logic gate may be located in a power island of theintegrated circuit. The adaptive leakage controller determines whetherto adjust the negative voltage to minimize the static leakage. Theadaptive leakage controller may continuously or periodically determinewhether to adjust the negative voltage. The negative voltage regulatoradjusts the negative voltage depending on the determination.

A method for minimizing static leakage of the integrated circuitcomprises generating the negative voltage, applying the negative voltageto the sleep transistor, determining whether to adjust the negativevoltage to minimize the static leakage, and adjusting the negativevoltage depending on the determination. The method may comprisecontrolling static leakage of the logic gate of the integrated circuitwith the sleep transistor. The method may comprise monitoring one ormore parameters of the sleep transistor.

In at least one example embodiment, the adaptive leakage controllerdetermines whether to adjust the negative voltage, and therefore staticleakage is minimized with changes in operating temperature of theintegrated circuit, or with voltage fluctuations or manufacturingvariations. Rather than a fixed negative voltage, the negative voltageapplied to the sleep transistor is adjusted to minimize the staticleakage. A further advantage is that single threshold transistorcircuitry may be utilized in the integrated circuit, reducing thecomplexity of the manufacturing process for the integrated circuit. Astill further advantage is that the negative voltage may be generatedwithin the integrated circuit, obviating components external to theintegrated circuit for generating the negative voltage.

According to one example embodiment, there is an integrated circuit thatincludes two power supply terminals for powering the integrated circuit.The power supply terminals include a V_(dd) positive supply terminal anda V_(ss) ground terminal together defining a range of logic levels. Theintegrated circuit also includes logic components. Each of the logiccomponents is a selected one of a logic gate and a storage cell, andeach of the logic components includes a sleep transistor in series witheach electrical connection to one of the power supply terminals. Avoltage generator generates a voltage outside the range of logic levels.The integrated circuit also includes circuitry for applying the voltageoutside the range of logic levels to the sleep transistor during a powerdown mode. In a mode other than power down mode, the circuitry may applyV_(dd) to the sleep transistor. Alternatively, the circuitry may apply avoltage greater than V_(dd) to the sleep transistor when in a mode otherthan power down mode. The integrated circuit also includes a voltageregulator for controlling the voltage generator to adequately minimizeleakage current through the sleep transistor during the power down mode.

According to one example embodiment, there is an integrated circuit thatincludes two power supply terminals for powering the integrated circuit.The power supply terminals include a V_(dd) positive supply terminal anda V_(ss) ground terminal. The integrated circuit also includes logiccomponents. Each of the logic components is a selected one of a logicgate and a storage cell, and each of the logic components includes asleep transistor in series with each electrical connection to one of thepower supply terminals. A charge pump generates a negative voltage. Theintegrated circuit also includes circuitry for applying the negativevoltage to the sleep transistor during a power down mode. The integratedcircuit also includes a voltage regulator for controlling the chargepump to adequately minimize leakage current through the sleep transistorduring the power down mode.

According to yet another example embodiment, there is a leakage managersystem for adequately minimizing static leakage of an integratedcircuit. The leakage manager system includes a generator configured togenerate a control signal to be applied to a sleep transistor. A monitoris configured to determine whether to adjust the control signal toadequately minimize the static leakage. The monitor includes an emulatedsleep transistor. A regulator is configured to adjust the control signaldepending on the determination.

According to yet another example embodiment, there is a method foradequately minimizing static leakage of an integrated circuit havinglogic components. Each of the logic components is a selected one of alogic gate and a storage cell, and each of the logic components includesa sleep transistor in series with each electrical connection to a V_(ss)ground terminal. The method includes generating a negative voltage to beapplied to the sleep transistor. The method also includes determiningwhether to adjust the negative voltage to adequately minimize the staticleakage. The method also includes adjusting the negative voltagedepending on the determination.

According to yet another example embodiment, there is an adaptiveleakage controller for adequately minimizing a static leakage of anintegrated circuit. A capacitor is configured to be charged to apositive supply voltage. A transistor is configured to discharge thecapacitor at a rate in proportion to the static leakage. A controlcircuit is configured to determine whether to adjust a negative voltageapplied to a sleep transistor configured to control the static leakagebased on a minimum rate of discharge of the capacitor.

According to yet another example embodiment, there is a method foradequately minimizing static leakage of an integrated circuit. Themethod includes charging a capacitor to a positive supply voltage, andalso discharging the capacitor at a rate in proportion to the staticleakage. The method also includes adjusting a negative voltage appliedto a gate of a sleep transistor to adequately minimize the rate ofdischarge of the capacitor.

According to yet another example embodiment, there is a power managementmethod carried out in an integrated circuit having logic components, aV_(dd) positive supply terminal and a V_(ss) ground terminal. Each ofthe logic components includes a sleep transistor in series with eachelectrical connection to one of the terminals. The V_(dd) positivesupply terminal and the V_(ss) ground terminal define a range of logiclevels. The method includes generating a voltage outside the range oflogic levels, and also applying the generated voltage outside the rangeof logic levels to the sleep transistor during a power down mode. Themethod also includes adjusting the generated voltage to adequatelyminimize leakage current through the sleep transistor during the powerdown mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an integrated circuit embodying a systemfor minimizing static leakage, in accordance with an example embodiment;

FIG. 2 is an illustration of a sleep transistor for minimizing staticleakage of the logic gate of FIG. 1, in accordance with an exampleembodiment;

FIG. 3 is an illustration of a graph of static leakage of the logic gateof FIG. 2, for a range of negative voltage at the gate of the sleeptransistor, in accordance with an example embodiment;

FIG. 4 is a block diagram of the leakage manager system for minimizingstatic leakage of the logic gate by application of the negative voltageof to the sleep transistor of FIG. 2, in accordance with an exampleembodiment;

FIG. 5 is an illustration of a method to minimize the static leakage ofthe logic gate of FIG. 2, in accordance with an example embodiment;

FIG. 6 is an illustration of the adaptive leakage controller (ALC) ofFIG. 4, in accordance with an example embodiment;

FIG. 7 is an illustration of the ALC of FIG. 4, in accordance with analternative example embodiment;

FIG. 8 is an illustration of a method for minimizing static leakage ofthe logic gate of FIG. 2, in accordance with the embodiment of the ALCof FIG. 7;

FIG. 9 is an illustration of the negative voltage regulator of FIG. 4for minimizing static leakage of the logic gate, in accordance with anexample embodiment; and

FIG. 10 is an illustration of the charge pump of FIG. 4 for minimizingstatic leakage, in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in the exemplary drawings wherein like reference numeralsindicate like or corresponding elements among the figures, exampleembodiments of a system and method according to the present inventionare described below in detail. It is to be understood, however, that thepresent invention may be embodied in various forms. For example,although described herein as pertaining to minimizing static leakage ofan integrated circuit, aspects of the invention may be practiced oncircuitry not embodied within an integrated circuit. Therefore, specificdetails disclosed herein are not to be interpreted as limiting, butrather as a basis for the claims and as a representative basis forteaching one skilled in the art to employ the present invention invirtually any appropriately detailed system, structure, method, processor manner.

FIG. 1 is a block diagram of an integrated circuit 100 embodying asystem for minimizing static leakage, in accordance with an exampleembodiment. The integrated circuit 100 is any electronic device that isinstantiated into silicon and/or similar manufacturing materials. Oneexample of the integrated circuit 100 is a system-on-a-chip. Theintegrated circuit 100 includes multiple intellectual property (IP)units, which are blocks of circuitry performing specific functions. Itwill be appreciated that functions of the integrated circuit 100described herein may be performed by a single integrated circuit 100 ormay be partitioned among several integrated circuits 100. The exemplaryintegrated circuit 100 of FIG. 1 includes a central processor unit (CPU)105, one or more power islands 110, one or more power island managers120, and one or more leakage manager systems 130.

While FIG. 1 depicts one power island 110 and one power island manager120 for the sake of simplicity, other embodiments of the integratedcircuit 100 may include any number of power islands 110, power islandmanagers 120, and leakage manager systems 130. In such embodiments, someof the power islands 110 may comprise different circuitry with respectto other power islands 110. The power island 110 and the power islandmanager 120 are further described in co-pending U.S. patent applicationSer. No. 10/840,893, entitled “Managing Power on Integrated CircuitsUsing Power Islands,” filed May 7, 2004.

The power island 110 is any section, delineation, partition, or divisionof the integrated circuit 100 in which power consumption is controlled.In some embodiments, multiple power islands 110 are delineated based ongeographical factors of the integrated circuit 100. In some embodiments,multiple power islands 110 are delineated based on functional IP unitsof the integrated circuit 100. In some embodiments, the power island 110comprises sub-islands of power to provide further specificity incontrolling power in the integrated circuit 100. In some embodiments,each of multiple power islands 110 includes power control circuitry tocontrol power within the power island 110.

The power island manager 120 is any circuitry, device, or system todetermine a target power level for one of the power islands 110,determine an action to change a consumption power level of the one ofthe power islands 110 to the target power level, and perform the actionto change the consumption power level of the one of the power islands110 to the target power level. The power island manager 120 can thusdynamically change the power consumption of the power islands 110 basedon the needs and operation of the integrated circuit 100. The targetpower level is a desired, calculated, or specified power consumption ofthe power islands 110. The power island manager 120 may be a hierarchyor group of power island managers 120.

While FIG. 1 depicts one leakage manager system 130 coupled to one powerisland manager 120 for the sake of simplicity, some embodiments comprisea plurality of leakage manager systems 130. In certain embodimentsincluding a plurality of leakage manager systems 130, each of theleakage manager systems 130 is coupled to one of a plurality of powerisland managers 120. In some embodiments, functions of the leakagemanager system 130 are distributed. In some embodiments, a singleleakage manager system 130 is coupled to one or more power islandmanagers 120. It will be appreciated that principles of the inventionmay apply to a circuit without power islands 110 or power islandmanagers 120.

The power island 110 includes one or more logic gates 115. In anembodiment without the power island 110, the logic gate 115 may compriseany logic gate of the integrated circuit 100. The logic gate 115 of theexemplary embodiment comprises any logic circuitry such as an inverter,a NAND, NOR, exclusive-OR, and exclusive-NOR gate, as well as a storagecells such as a flip-flop and a latch. The logic gate 115 may comprisehigher-level Boolean logic, including combinations of individual logicgates.

The logic gate 115 may be powered down to a “sleep mode” in conjunctionwith a sleep transistor (not shown), as described further herein. Tominimize static leakage of the logic gate 115, the leakage managersystem 130 generates a negative voltage 150 to be applied to the sleeptransistor. Applying the negative voltage 150 to a gate of an NMOS sleeptransistor coupled between the logic gate 115 and ground may reduce thestatic leakage of the logic gate 115. The leakage manager system 130receives a negative voltage enable signal 140 and subsequently generatesand transmits the negative voltage 150 to the power island 110. Thenegative voltage enable signal 140 may include other signals in additionto the negative voltage enable signal 140. The leakage manager system130 determines whether to adjust the negative voltage 150. Based on thedetermination, the leakage manager system 130 adjusts the negativevoltage 150, as described further herein.

Adjusting the negative voltage 150 applied to the sleep transistorminimizes static leakage of the logic gate 115. For example, staticleakage varies based on parameters such as operating temperature,voltage fluctuations, and manufacturing variations. Therefore,application of a fixed negative voltage to the sleep transistor does notoptimally minimize the static leakage of the logic gate 115.Furthermore, generating the negative voltage 150 “on chip” reducescomponent requirements external to the integrated circuit 100.

An alternative to reduce the static leakage of the logic gate 115comprises multiple threshold voltage CMOS, in which one or more highthreshold transistors are inserted in series with a low threshold logicgate 115. Switching the high threshold transistor “off” reduces thestatic leakage of the logic gate 115. However, the high thresholdtransistor requires extra manufacturing process steps for the integratedcircuit 100 and slows down the speed of the logic gate 115 as comparedto nominal threshold transistors. Providing the negative voltage 150 toa low threshold NMOS sleep transistor advantageously eliminates arequirement to provide high threshold sleep transistor, thereby reducingprocessing steps needed to manufacture the integrated circuit 100.

FIG. 2 is an illustration of a sleep transistor 210 for minimizingstatic leakage of the logic gate 115 of FIG. 1, in accordance with anexample embodiment. In some embodiments, the sleep transistor 210comprises an NMOS transistor cascaded in series with the logic gate(e.g., inverter) 115. Static leakage of the logic gate 115 passesthrough the sleep transistor 210 as a drain-source current (depicted asI_(d)) and/or as a drain-gate current (depicted as I_(g)). The staticleakage of the logic gate 115 equals I_(d)+I_(g) through the sleeptransistor 210. The negative voltage (SLPB) 150 applied to the sleeptransistor 210 may be used to control the static leakage of the logicgate 115 by regulating the drain-source current and the drain-gatecurrent of the sleep transistor 210.

FIG. 3 is an illustration of a graph of static leakage of the logic gate115 of FIG. 2, for a range of negative voltage at the gate of the sleeptransistor 210, in accordance with an example embodiment. As thenegative voltage (SLPB) 150 applied to the gate of the sleep transistor210 becomes increasingly negative, the drain-source current I_(d) of thesleep transistor 210 decreases. However, as the magnitude of thenegative voltage 150 increases beyond a minimum leakage point A, forexample to point B, the drain-gate current I_(g) of the sleep transistor210 exceeds the drain-source current I_(d). As a result, the staticleakage of the logic gate 115 increases. Accordingly, adjusting thenegative voltage 150 to approximately V(A), corresponding to asubstantial equality between the drain-source current I_(d) and thedrain-gate current I_(g) at the minimum leakage point A, minimizesstatic leakage in the logic gate 115.

FIG. 4 is a block diagram of the leakage manager system 130 forminimizing static leakage of the logic gate 115 by application of thenegative voltage of to the sleep transistor 210 of FIG. 2, in accordancewith an example embodiment. The leakage manager system 130 comprises anadaptive leakage controller (ALC) 410, a negative voltage regulator 420,and a charge pump 430. The charge pump 430 generates the negativevoltage 150 (SLPB). The ALC 410 determines whether to adjust thenegative voltage 150. The ALC 410 generates a signal (depicted as CTRL)depending on the determination. The negative voltage regulator 420adjusts the negative voltage 150 depending on the CTRL signal.

As described further herein, the negative voltage regulator 420 of oneembodiment generates an enable (EN) signal to the charge pump 430 toenable the charge pump 430 to increase the magnitude of the negativevoltage 150 (i.e., to make the negative voltage 150 more negative). Ifthe EN signal is low, an alternating signal from an oscillator 425 tothe charge pump 430 is disabled, preventing the charge pump 430 fromincreasing the magnitude of the negative voltage 150. Alternatively, ifthe EN signal is high, the alternating signal from the oscillator 425 isenabled so that the charge pump 430 will increase the magnitude of thenegative voltage 150. Because the negative voltage regulator 420 togglesthe EN signal on or off depending on whether the ALC 410 determines toadjust the negative voltage 150, the leakage manager system 130maintains the negative voltage 150 at a particular negative voltage tominimize static leakage of the logic gate 115.

FIG. 5 is an illustration of a method to minimize the static leakage ofthe logic gate 115 of FIG. 2, in accordance with an example embodiment.At step 500, the CPU 105 (FIG. 1) enters sleep mode. At step 510, thecharge pump 430 (FIG. 4) generates the negative voltage 150. At step515, the charge pump 430 applies the negative voltage 150 to the sleeptransistor 210 (FIG. 2). At step 520, the ALC 410 (FIG. 4) may monitorone or more parameters of the sleep transistor 210 corresponding to thestatic leakage of the logic gate 115. The ALC 410 may monitor the sleeptransistor 210 directly, or may monitor one or more emulated sleeptransistors, as described further with respect to FIGS. 6-8.

At step 530, the ALC 410 determines whether to adjust the negativevoltage 150 to minimize static leakage. If the ALC 410 determines toadjust the negative voltage 150, the ALC 410 generates the CTRL signalto the negative voltage regulator 420 (FIG. 4). At step 540, thenegative voltage regulator 420 adjusts the negative voltage 150 based onthe CTRL signal.

In one embodiment, the negative voltage regulator 420 continuouslyadjusts the negative voltage 150. In another embodiment, the negativevoltage regulator 420 periodically adjusts the negative voltage 150.

The leakage manager system 130 adjusts the negative voltage 150 tominimize the static leakage of the logic gate 115, even if the staticleakage varies due to effects such as temperature variation, voltagefluctuation, or manufacturing process variation. The leakage managersystem 130 may advantageously be wholly integrated into the integratedcircuit 100, obviating components external to the integrated circuit 100to generate the negative voltage 150. Further, the leakage managersystem 130 may advantageously be utilized in the integrated circuit 100comprising single threshold transistor logic, so that manufacturing ofthe integrated circuit 100 is simplified.

FIGS. 6-10 illustrate further detail of embodiments of the leakagemanager system 130 of FIG. 4.

FIG. 6 is an illustration of the adaptive leakage controller (ALC) 410of FIG. 4, in accordance with an example embodiment. The ALC 410 of thisembodiment comprises a first emulated sleep transistor 610, a secondemulated sleep transistor 620, a differential (operational) amplifier630, bias transistors 640, and a voltage offset transistor 650. It willbe appreciated that the ALC 410 of this embodiment comprises analogcircuitry to continuously determine whether to adjust the negativevoltage 150 of FIG. 4.

It will also be appreciated that although FIG. 6 depicts the biastransistors 640 as PMOS transistors with gate connected to drain toprovide a resistive voltage drop across the bias transistors 640, thebias transistors 640 may comprise resistors. In the exemplary embodimentwith PMOS bias transistors 640, matching between the several biastransistors 640 ensures substantially identical operation of the biastransistors 640. The voltage offset transistor 650 of the exemplaryembodiment similarly comprises a PMOS transistor with gate connected todrain to provide a resistive voltage drop across the voltage offsettransistor 650. Alternatively, the voltage offset transistor 650 maycomprise a resistor.

In FIG. 6, the negative voltage 150 (SLPB) is applied to a gate of thefirst emulated sleep transistor 610. The negative voltage 150correspondingly produces a first current through the first emulatedsleep transistor 610. The first current may comprise drain-gate currentand/or drain-source current. The first current through the firstemulated sleep transistor 610 is in proportion to the static leakage ofthe logic gate 115. The first current creates a first voltage dropacross the bias transistors (resistances) 640 at a drain of the firstemulated sleep transistor 610. The first voltage drop is sensed at anegative input of the differential amplifier 630.

With respect to the second emulated sleep transistor 620, the resistanceof the voltage offset transistor 650 reduces the magnitude of thenegative voltage 150 (SLPB) by a voltage offset. A gate of the secondemulated sleep transistor 620 receives the negative voltage 150 plus thevoltage offset. The negative voltage 150 plus the voltage offsetproduces a second current through the second emulated sleep transistor620. The second current may comprise drain-gate current and/ordrain-source current. The second current creates a second voltage dropacross the bias transistors (resistors) 640 at a drain of the secondemulated sleep transistor 620. The second voltage drop is sensed at apositive input of the differential amplifier 630.

In operation, the gate of the second emulated sleep transistor 620operates at a slight voltage offset as compared to the gate of the firstemulated sleep transistor 610, because of the voltage offset transistor650. Referring to FIG. 3, the voltage offset may be represented by thevoltage offset between points A and B, or V(B)-V(A). As a result of thevoltage offset, the minimum leakage point A may be detected by adjustingthe negative voltage 150 so that I(B) is substantially equal to I(A). Itwill be appreciated that operating parameters of the voltage offsettransistor 650 influence the magnitude of the voltage offset. Theoperating parameters may be based on such considerations as noise on thenegative voltage 150, for example.

In principle of operation with respect to FIG. 3, if the magnitude ofthe negative voltage 150 produces a first current I(B) in the firstemulated sleep transistor 610 corresponding to point B, and the negativevoltage 150 plus the voltage offset produces a second current I(A) inthe second emulated sleep transistor 620 corresponding to point A, thenthe differential amplifier 630 will generate the CTRL signal so that themagnitude of the negative voltage 150 will be adjusted until I(A)substantially equals I(B). Alternatively, if the negative voltage 150 issuch that the first emulated sleep transistor 610 and the secondemulated sleep transistor 620 produce substantially equal currents, sothat I(A)=I(B), then the differential amplifier 630 will maintain thepresent value of the CTRL signal. The resulting operating point will bea negative voltage which is offset from the ideal operating point by avalue equal to one half the voltage offset produced by the currentthough the voltage offset transistor 650. If gate leakage is negligible,there may be no inflection in the leakage vs. gate voltage curve of FIG.3. In this case, the CTRL signal will decrease to its minimum value,causing the charge pump 430 (FIG. 4) to operate at its most negativevoltage.

In conjunction with the negative voltage regulator 420 of FIG. 9, theALC 410 of this embodiment advantageously minimizes static leakage ofthe logic gate 115 by continuously controlling the negative voltage 150to approximately the minimum leakage point A of FIG. 3.

FIG. 7 is an illustration of the ALC 410 of FIG. 4, in accordance withan alternative example embodiment. The ALC 410 of this embodimentcomprises a charging transistor 710, a capacitor 715, an emulated sleeptransistor 720, a comparator 730, a counter 740, and a register 750. Thecharging transistor 710 is switched by a controller (not shown) tocharge the capacitor 715 to a positive supply voltage (e.g., V_(DD)).The controller may also switch the charging transistor 710 so that thecapacitor 715, once charged, may discharge through the emulated sleeptransistor 720. The comparator 730, the counter 740, and the register750 comprise a control circuit to measure a time needed to discharge thecapacitor 715 to a predetermined value VREF. A state logic machine (notshown) coupled to the register 750 may compare values stored in theregister 750, as described with respect to FIG. 8.

In this embodiment of the ALC 410, the maximum discharge time for thecapacitor 715 corresponding to the lowest value of static leakage isused to generate a digital value for the CTRL signal to the negativevoltage regulator 420 (FIG. 4). The ALC 410 periodically updates theCTRL signal if the ALC 410 determines to adjust the negative voltage150. The operation of the ALC 410 of this embodiment is described withrespect to FIG. 8.

FIG. 8 is an illustration of a method for minimizing static leakage ofthe logic gate 115 of FIG. 2, in accordance with the embodiment of theALC 410 of FIG. 7. In overview, the method comprises charging thecapacitor 715 to the positive supply voltage V_(DD), discharging thecapacitor at a rate in proportion to the static leakage of the logicgate 115 via the emulated sleep transistor 720, and adjusting thenegative voltage 150 to minimize the rate of discharge of the capacitor715. The negative voltage 150 that corresponds to minimum currentthrough the emulated sleep transistor 720 (i.e., minimum static leakage)minimizes the discharge rate of the capacitor 715 and maximizes the timeto discharge the capacitor 715.

At step 805, the CTRL signal is initialized to its minimum value.Setting the CTRL signal to its minimum value directs the negativevoltage regulator 420 to drive the magnitude of the sleep signal SLPB150 to its minimum value. At step 810, the controller switches thecharging transistor 710 so that the capacitor 715 is charged to V_(DD).At step 815, the charging transistor 710 is switched off so that thecapacitor 715 may discharge through the emulated sleep transistor 720.At step 820, the reference voltage VREF is set to a constant voltagewhich is less than V_(DD) (e.g. V_(DD)/2). At step 825, the comparator730 generates an output to the counter 740 after the capacitor 715discharges to VREF. The counter 740 determines a time required todischarge the capacitor 715 to VREF. The register 750 stores a count(i.e., time) of the counter 740.

At step 827, the CTRL signal is incremented by one bit. At step 830, thecontroller switches the charging transistor 710 so that the capacitor715 is again charged to V_(DD). At step 840, the charging transistor 710is switched off. At step 860, the comparator 730 generates an output tothe counter 740 after the capacitor 715 discharges to VREF. The counter740 determines the time required to discharge the capacitor 715 with thenew value of the CTRL signal and the corresponding SLPB signal.

At step 870, the state logic machine compares the value of the register750 for the current pass through steps 830-860 (i.e., the time requiredto discharge the capacitor 715 to VREF for the new value of the CTRLsignal and the SLPB signal) to the value of the register 750 for theprevious pass through steps 830-860. If the value of the register 750for the current pass did not decrease relative to the value of theregister 750 for the previous pass, then the new value of the CTRLsignal corresponds to a lower value of static leakage through theemulated sleep transistor 720. In this case, the method returns to step827 to further increment the CTRL signal and measure the time requiredto discharge the capacitor 715. Alternatively, at step 870, if the timerequired to discharge the capacitor 715 decreased in the current pass,corresponding to a higher value of static leakage through the emulatedsleep transistor 720, then the previously stored value of the register750 corresponds to the lowest value of static leakage through theemulated sleep transistor 720. The value of the CTRL signalcorresponding to minimal static leakage is used to control the negativevoltage regulator 420 to generate the appropriate setting for thenegative voltage 150.

One advantage of the embodiment of the digital ALC 410 of FIGS. 7-8 isthat the CTRL signal comprises a digital signal. The digital CTRL signalmay be routed via the control signal 140 to multiple leakage managers130 of FIG. 1. For example, because silicon is an excellent thermalconductor, it may be advantageous to utilize a single digital ALC 410with leakage managers 130 and power island managers 120. Each of themultiple power island managers 120 of this embodiment comprise thenegative voltage regulator 420 and the charge pump 430, so that thefunctions of the leakage controller system 130 may be distributed asneeded across the integrated circuit 100.

FIG. 9 is an illustration of the negative voltage regulator 420 of FIG.4 for minimizing static leakage of the logic gate 115, in accordancewith an example embodiment. The negative voltage regulator 420 includesan interface to receive the negative voltage 150, a first voltagedivider 905, a second voltage divider 915, and a comparator 920. In oneembodiment, the first voltage divider 905 comprises a series of stackedPMOS transistors (not shown) with bulk tied to source. It will beappreciated, for example, that a series of three equivalent stacked PMOStransistors with bulk tied to source provide a divide-by-3 voltagedivider in the first voltage divider 905. It will further be appreciatedthat the first voltage divider 905 may comprise any ratio of division.The first voltage divider 905 provides a fixed voltage reference (e.g.,point C) with respect to a positive voltage source (e.g., V_(DD)). Thefixed voltage reference of this embodiment is coupled to a negativeterminal of the comparator 920.

Similarly, a series of three equivalent stacked PMOS transistors withbulk tied to source provide a divide-by-3 voltage divider in the fixedresistances of the second voltage divider 915. It will be appreciatedthat the second voltage divider 915 may comprise any ratio of division.The second voltage divider 915 of this embodiment is coupled to apositive terminal of the comparator 920.

In an embodiment in conjunction with the analog CTRL signal generated bythe ALC 410 of FIG. 6, a variable resistor 910 of the second voltagedivider 915 allows the second voltage divider 915 to generate a variablevoltage reference (e.g., point D) depending on the negative voltage 150and a received signal (CTRL) generated by the ALC 410. The variableresistor 910 may comprise a transistor circuit. Depending on the CTRLsignal, the variable resistor 910 varies between high impedance and lowimpedance.

In conjunction with the digital ALC 410 of FIGS. 7-8, the variableresistor 910 of the second voltage divider 915 comprises a switchedresistor network controlled by the digital CTRL signal. The variableresistor 910 of this embodiment may comprise two or more switchedresistors. The variable resistor 910 may also comprise two or more PMOStransistors with bulk tied to source.

In operation, the negative voltage regulator 420 adjusts the negativevoltage 150 depending on a comparison between the fixed voltagereference (point C) and the variable voltage reference (point D). Thecomparator 920 may generate an enable (EN) signal to enable the chargepump 430 (FIG. 4) to increase the magnitude of the negative voltage 150.If the EN signal is low, the alternating signal from the oscillator 425(FIG. 4) to the charge pump 430 is disabled, preventing the charge pump430 from increasing the magnitude of the negative voltage 150. If the ENsignal is high, the alternating signal from the oscillator 425 isenabled so that the charge pump 430 will increase the magnitude of thenegative voltage 150. Therefore, depending on the CTRL signal from theALC 410, the comparator 920 will control the charge pump 430 to increasethe magnitude of the negative voltage 150 or allow it to decrease.

FIG. 10 is an illustration of the charge pump 430 of FIG. 4 forminimizing static leakage, in accordance with various embodiments of theinvention. The charge pump 430 may receive and function to increase themagnitude of the SLPB signal 150 (as discussed in FIG. 4). The output ofthe charge pump 430 may be V_(SS) (see FIG. 10) which, in variousembodiments, functions as the SLPB signal 150 to be applied to the sleeptransistor and/or the power island 110. The charge pump 430 may alsoreceive alternating signals from the oscillator 425 as either the INPsignal or the INN signal (in some embodiments, the INN signal is aninverted (i.e., a complement of the) INP signal). Further, the chargepump 430 may receive an EN signal (discussed in FIG. 4) which may enableand/or disable the charge pump 430. The EN signal may be received by thecharge pump 430 as the SLP signal (see FIG. 10).

The charge pump 430 comprises two interfaces for voltage (e.g., V_(DD)line 1002 and V_(SS) line 1004), an input for an alternating signal(i.e., an INP line 1006), an input for an inverted alternating signal(i.e., an INN line 1008), an inverter 1010, a pump capacitor 1012,capacitances 1014 and 1016, a cross-coupled pass gate 1018 and 1020,PMOS transistors 1022 and 1024, node 1026, an SLP line 1028, an inverter1030, and an SLPB line 1032. The cross-coupled pass gate 1018 maycomprise two PMOS transistors 1038 and 1040. The cross-coupled pass gate1020 may comprise two PMOS transistors 1042 and 11044. The inverter 1010may comprise a NMOS transistor 1034 and a PMOS transistor 1036.

In example embodiments, the capacitance 1014 is electrically coupled toINP line 1006 and the capacitance 1016 is electrically coupled to theINN line 1008. The capacitance 1014 and 1016 may comprise a capacitorsuch as a metal-metal capacitor. In other embodiments, the capacitance1014 and 1016 may comprise PMOS capacitances (e.g., varactors).Alternately, the capacitance 1014 and 1016 may comprise similar ordifferent components. Those skilled in the art will appreciate that thecapacitance 1014 and 1016 may be many different components comprisingcapacitances. In various embodiments, the capacitances 1014 and 1016function to smooth out transients from the INP signals and the INNsignals, respectively.

The gate of PMOS transistors 1022 and 1024 may be electrically coupledto the capacitance 1014 and 1016, respectively. The PMOS transistor 1022and PMOS transistor 1024 may be electrically coupled to the pumpcapacitor 1012. The PMOS transistor 1022 may also be electricallycoupled to the NMOS transistor 1034 within inverter 1010 as well as theV_(SS) line 1004, the gate of the PMOS transistor 1038 in the crosscoupled pass gate 1018, and the gate of the PMOS transistor 1044 in thecross coupled pass gate 1020. PMOS transistor 1024 may be coupled toSLPB line 1032. In various embodiments, the substrates of PMOStransistor 1022 and 1024 are electrically coupled to node 1026.

The output of the inverter 1010 is electrically coupled to the pumpcapacitor 1012. The drain of PMOS transistor 1036 is coupled to thesource of NMOS transistor 1034 as well as the pump capacitor 1012. TheINP line 1006 is electrically coupled to the gates of both the PMOStransistor 1036 and the NMOS transistor 1034 (e.g., the INP line 1006 iselectrically coupled to the input of the inverter 1010).

The cross-coupled pass gate 1018 may comprise two PMOS transistors 1038and 1040. In one example, the PMOS transistor 1038 is electricallycoupled to the capacitance 1014, the gate of PMOS transistor 1022, thePMOS transistor 1040, and the gate of PMOS transistor 1042 in thecross-coupled pass gate 1020. The substrate and drain of PMOS transistor1038 may be electrically coupled to the substrate and drain of the PMOStransistor 1040 as well as the node 1026. The gate of PMOS transistor1040 is electrically coupled to the PMOS transistors 1042 and 1044 aswell as the capacitance 1016 and the gate of PMOS transistor 1024.

The cross-coupled pass gate 1020 may comprise two PMOS transistors 1042and 1044. In one example, the substrate of the PMOS transistor 1042 iselectrically coupled to the substrate of PMOS transistor 1044 and thenode 1026. The PMOS transistor 1042 and the PMOS transistor 1044 areelectrically coupled to the node 1026.

The cross-coupled pass gate 1018 of this embodiment may be capacitivelycoupled to the alternating signal (the INP signal) from the oscillator425 (FIG. 4). The cross-coupled pass gate 1020 may be capacitivelycoupled to a complement of the alternating signal (the INN signal) fromthe oscillator 425. The V_(SS) (over the V_(SS) line 1004) may supplynegative voltage to the sleep transistor 210 to control the staticleakage of the logic gate 115 of FIG. 2.

The V_(DD) line 1002, V_(SS) line 1004, INP line 1006, INN line 1008,and SLPB line 1032, and SLP line 1028 may comprise wires, traces, or anyconductive material configured to function as an electrical medium. TheINP line 1006 may be coupled with the oscillator 425 which may generatean alternating signal (i.e., the INP signal). The INN line 1008 may becoupled with an inverter configured to invert the alternating signal(i.e., the INP signal) to generate a complement of the alternatingsignal. It will be appreciated by those skilled in the art that, in someembodiments, the INN line 1008 receives an alternating signal and theINP line 1006 receives the complement of the alternating signal. Theremay be many ways to generate the alternating signal and/or thecomplement of the alternating signal.

Further, the SLPB line 1032 may receive the sleep signal from theleakage manager system 130. In various embodiments, the sleep signal isa negative voltage signal and the SLPB line 1032 is a negative voltageline. The SLP line 1028 may receive the SLP signal (e.g., the enable(EN) signal) from the negative voltage regulator 420. There may be manyways in which the SLP signal may be generated. Further, the SLP signalmay be generated in such a way as to make the inversion of the signaleither optional or unnecessary (i.e., the inverter 1030 may beoptional).

In various embodiments, the alternating signal (INP signal) and thecomplement of the alternating signal (INN signal) may each comprise twostates discussed herein including “high” and “low.” Those skilled in theart will appreciate that the “high” signal is “high” when compared tothe “low” state of the signal and is not “high” or “low” in comparisonwith another standard. In one example, the high state is 1 volt and thelow state is 0 or −1 volts. As used herein, the high state is referredto as “high” and the low state is referred to as “low.”

In various embodiments, when the INP signal is low (or goes low), thecharge within the pump capacitor 1012 is released through the V_(SS)signal (via V_(SS) line 1004). In one example, the INP signal isreceived over the INP line 1006 by the gates of the inverter 1010 (i.e.,the gate of the PMOS transistor 1036 and the gate of the NMOS transistor1034). When the INP signal is low (or goes to low), the V_(DD) signalmay pass through from the source of the PMOS transistor 1036 to the pumpcapacitor 1012. Similarly, the INP signal is received by capacitance1014 and, subsequently, the gate of PMOS transistor 1022. As a result,the charge of the pump capacitor 1012 may be released through the PMOStransistor 1022 and out through the V_(SS) line 1004. The alternate ofthe INP signal, the INN signal, which is high (or goes to high), iscoupled to the capacitance 1016 over the INN line 1008. The gate of PMOStransistor 1024 may receive the high signal from the capacitance 1016.As a result, the PMOS transistor 1024 may decouple the SLPB line 1032from the pump capacitor 1012.

When the INP signal is high (or goes high), the pump capacitor 1012 ischarged (i.e., the capacitor is charged by receiving the V_(SS) signaland the SLPB signal). When the INP signal is high (or goes to high), thePMOS transistor 1036 no longer allows the pump capacitor 1012 to receivethe V_(DD) signal. The gate of NMOS transistor 1034 receives the INPsignal over the INP line 1006 which subsequently allows the pumpcapacitor 1012 to receive the V_(SS) signal from V_(SS) line 1004 (theINP signal (i.e., high or going to high) is received by the gate of thePMOS transistor 1022 which prevents the V_(SS) signal from flowingthrough the PMOS transistor 1022). The alternate of the INP signal, theINN signal (i.e., which is low or goes to low) is received by the gateof PMOS transistor 1024 which subsequently allows the SLPB signal (viathe SLPB line 1032) to be received by the pump capacitor 1012 therebyallowing the pump capacitor 1012 to charge.

In some embodiments, the node 1026 is simply tied to ground. In otherembodiments, the node 1026 is not tied to ground, but is coupled to theSLP signal. In one example, the SLP signal (via the SLP line 1028) iselectrically coupled to the input of inverter 1060, the output of whichis coupled to the node 1026. The inverter 1030 may be activated onexiting the sleep mode to prevent a power supply that generates V_(DD)from being shorted to ground through the PMOS transistors 1022 and 1024,and may ensure that any P-N junctions in the wells are not forwardbiased.

In various embodiments, there is no current flow from the PMOStransistors to the substrate, since the substrate may be at an equal orhigher potential than the source and drain of the PMOS transistors. Inone example, current flow from the PMOS transistors to the substrate isavoided in order to compete against forward biased diodes for currentflow. In another example, to ensure that no P-N junctions in the wellsof the PMOS transistors are forward biased, the inverter 1030 may outputa complement of the activated SLP signal to drive the node 1026 to 0 V.

The SLP signal may disable the charge pump 430. In one example, the SLPsignal (e.g., the EN signal in FIG. 4) goes low. The node 1026 receivesthe SLP signal via the SLP line 1028 over the inverter 1030, and, assuch, the node 1026 may receive a signal in a “high” state. The node1026 electrically couples the high signal to the body of PMOStransistors 1022, 1038, 1040, 1042, 1044, and 1024. As a result, thePMOS transistors 1022, 1038, 1040, 1042, 1044, and 1024 do not allowcurrent flow (e.g., are disabled) thereby disabling the charge pump 430.

Those skilled in art will appreciate that when either the INP signal orthe INN signal is high (or goes to high), the signal may electricallycouple to the node 1026, in various embodiments. In one example, the INPsignal is high and the INN signal is low. The low signal (via the INNline 1008 and the capacitance 1016) is received at the gate of PMOStransistor 1040 which may allow the high INP signal to flow through thePMOS transistor 1040 to the node 1026. In another example, the INNsignal is high and the INP signal is low. The low signal (via the INPline 1006 and the capacitance 1014) is received at the gate of PMOStransistor 1042 which may allow the high INN signal to flow through thePMOS transistor 1042 to the node 1026.

In various embodiments, the alternating connectivity of high signalswith the node 1026 allows the high signal current to drain at groundwhen ground is coupled to node 1026. Alternatively, the alternatingconnectivity of high signals with the node 1026 may electrically couplewith the SLP line 1028. In one example, the alternating high signalsreceived by the body of PMOS transistors 1022, 1038, 1040, 1042, 1044,and 1024 (via node 1026) prevent leakage from the pump capacitor 1012 orprevents the V_(DD) signal from coupling to ground. In another example,the alternating high signal over the node 1026 may reduce the voltagerequired by the SLP signal to sufficiently bias the bodies (i.e.,substrates) of the PMOS transistors 1022, 1038, 1040, 1042, 1044, and1024 in order to disable the charge pump 430.

While one skilled in the art should be able to implement and gain thebenefits of the charge pump 430 if provided with only the circuits anddiagrams of FIGS. 1-10, charging and discharging of the pump capacitor1012 will now be described so that functional aspects of other exampleembodiments of the invention, that are clear from the drawings, may beexplained in words that confirm what is shown in the drawings.

With reference to FIGS. 4 and 10, in some example embodiments, the INPsignal becomes ‘0’ and the INN signal becomes ‘1’ in response to therising edge of the oscillator 425. Due to the INP signal becoming ‘0’, avoltage drop exists across the capacitance 1014, so the gate of thefirst PMOS transistor 1022 and the gate of the transistor 1042 is at‘−1’. The cross-coupled pass gate 1020 is conducting while a negativevoltage is applied at the gate of the transistor 1042. The first PMOStransistor 1022 is also conducting as the negative voltage is applied atthe gate of the first PMOS transistor 1022. While the first PMOStransistor 1022 is on, the pump capacitor 1012 is discharging throughthe V_(SS) line 1004.

Due to the INN signal becoming ‘1’, there may be a positive voltage atthe capacitance 1016 because this terminal receives the INN signal. Witha ‘1’ at the first terminal of the capacitance 1016, there is a ‘0’ atthe second terminal of the capacitance 1016. In various embodiments, thegate of the PMOS transistor 1040 and the second terminal of thecapacitance 1016 share the same node, so the PMOS transistor 1040 isnon-conducting because the gate-to-source voltage difference (V_(GS)) isgreater than the threshold voltage (V_(T)). As a result, thecross-coupled pass gate 1018 is non-conducting during the dischargingphase. Further, the PMOS transistor 1024 will be off during thedischarging phase. As a result, charging of the pump capacitor 1012 doesnot occur during the discharging phase.

Next is the falling edge of the oscillator 425. In response, the INPsignal may go from ‘0’ to ‘1’, and, consequently, the first and secondterminals of the capacitance 1014 go from ‘0’ and ‘1’, respectively, to‘−1’ and ‘0’, respectively. The cross-coupled pass gate 1020 becomesnon-conducting because V_(GS) of the transistor 1042 will rise aboveV_(T) (i.e. the transistor 1042 will become non-conducting).

Also in response to the falling edge of the oscillator 425, the INNsignal goes from ‘1’ to ‘0’, and consequently the first and secondterminals of the capacitance 1016 go from ‘1’ to ‘0’ and ‘0’ to ‘−1’,respectively. So the node shared by the second terminal of thecapacitance 1016, the gate of the second PMOS transistor 1024 and thegate of the PMOS transistor 1040 will be at ‘−1’. The cross-coupled passgate 1018 will be conducting while a negative voltage is applied at thegate of the PMOS transistor 1040. The second PMOS transistor 1024 isalso conducting during this period of time, as the negative voltage isalso applied at the gate of the second PMOS transistor 1024. While thesecond PMOS transistor 1024 is on, the pump capacitor 1012 is charging.The PMOS transistor 1022 may be off during the above-described chargingphase, so, in the illustrated example embodiment, discharging of thepump capacitor 1012 does not occur during the charging phase.

With reference now to FIGS. 4-10, it will be understood that the leakagemanager system 130, comprising the adaptive leakage controller 410, thenegative voltage regulator 420, and the charge pump 430, minimizes thestatic leakage of the logic gate 115, even if the static leakage variesdue to effects such as temperature variation, voltage fluctuation, ormanufacturing process variation. The leakage manager system 130 may bewholly integrated into the integrated circuit 100, obviating componentsexternal to the integrated circuit 100. Further, the leakage managersystem 130 may advantageously be utilized in the integrated circuit 100comprising single threshold transistor logic, simplifying manufacturingof the integrated circuit 100.

Modification of the previously described charge pump 430 to make itsuitable for operation in different voltage ranges is contemplated. Forexample, a higher voltage (for instance, +2V) at the high end of thevoltage operation range may be possible by customizing the circuit byswitching the INN signal and the INP signal as well as using some biggercircuit components such as, for instance, bigger capacitors.

The components, type of components, and number of components identifiedin FIG. 10 are illustrative. For example, in some embodiments, thecharge pump 430 may not comprise the PMOS transistor 1044 and the PMOStransistor 1038. Further, the inverter 1030 and SLP signal may beoptional (e.g., the inverter 1030 and SLP signal may be replaced with aground or a wire coupled to ground).

Further, the above description is illustrative and not restrictive. Manyvariations of the invention will become apparent to those of skill inthe art upon review of this disclosure. The scope of the inventionshould, therefore, be determined not with reference to the abovedescription, but instead should be determined with reference to theappended claims along with their full scope of equivalents.

1. An adaptive leakage controller for adequately minimizing a staticleakage of an integrated circuit, comprising: a capacitor configured tobe charged to a positive supply voltage; a transistor configured todischarge the capacitor at a rate in proportion to the static leakage;and a control circuit configured to determine whether to adjust anegative voltage applied to a sleep transistor configured to control thestatic leakage based on a minimum rate of discharge of the capacitor. 2.The adaptive leakage controller of claim 1 wherein the control circuitcomprises: a variable reference voltage; and a measuring circuitconfigured to measure a time required to discharge the capacitor to alevel substantially equal to the variable reference voltage.
 3. Theadaptive leakage controller of claim 2 wherein the measuring circuitcomprises a counter.
 4. The adaptive leakage controller of claim 3further comprising: a register operably coupled to the counter.
 5. Theadaptive leakage controller of claim 4 further comprising: a state logicmachine operably coupled to the register.
 6. The adaptive leakagecontroller of claim 1 further comprising: another transistor operablycoupled to the capacitor and configured to charge the capacitor to apositive power supply.
 7. The adaptive leakage controller of claim 1wherein the control circuit is configured to periodically determinewhether to adjust the negative voltage.
 8. The adaptive leakagecontroller of claim 1 wherein the control circuit is configured tocontinuously determine whether to adjust the negative voltage.
 9. Theadaptive leakage controller of claim 1 wherein the control circuit isconfigured to generate a digital control signal based on the negativevoltage.
 10. The adaptive leakage controller of claim 1 wherein thenegative voltage is generated within the integrated circuit.
 11. Theadaptive leakage controller of claim 1 in combination with a charge pumpconfigured to generate the negative voltage.
 12. The adaptive leakagecontroller of claim 1 in combination with a negative voltage regulatorconfigured to adjust the negative voltage.
 13. The adaptive leakagecontroller and negative voltage regulator of claim 12, wherein thenegative voltage regulator is configured to generate a signal thatenables a change in the magnitude of the negative voltage.
 14. Theadaptive leakage controller of claim 1, wherein the adaptive leakagecontroller is integrated into the integrated circuit.
 15. A method ofadequately minimizing static leakage of an integrated circuitcomprising: charging a capacitor to a positive supply voltage;discharging the capacitor at a rate in proportion to the static leakage;and adjusting a negative voltage applied to a gate of a sleep transistorto adequately minimize the rate of discharge of the capacitor.
 16. Themethod of claim 15, wherein adjusting the negative voltage includesgenerating a digital control signal.
 17. The method of claim 15, furthercomprising measuring a time to discharge the capacitor to apredetermined value.
 18. The method of claim 15, further comprising:measuring a first time required to discharge the capacitor to a firstpredetermined value; measuring a second time required to discharge thecapacitor to a second predetermined value; and adjusting the negativevoltage depending on a comparison between the first and second times.19. The method of claim 18 further comprising: charging the capacitorbetween measuring the first and second times required to discharge thecapacitor.
 20. The method of claim 18 wherein measuring the first andsecond times includes storing first and second counts that correspond tothe first and second times, respectively.
 21. The method of claim 20wherein the comparison is a comparison of the first and second counts.22. The method of claim 21 wherein adjusting the negative voltagedepending on the comparison includes incrementing a digital controlsignal if the first count is smaller than the second count.
 23. Themethod of claim 22 wherein adjusting the negative voltage depending onthe comparison includes reverting to a prior value for a digital controlsignal if the first count is greater than the second count.
 24. Themethod of claim 15 wherein adjusting the negative voltage occursperiodically.
 25. The method of claim 15 wherein adjusting the negativevoltage occurs continuously.
 26. The method of claim 15 furthercomprising: generating the negative voltage within the integratedcircuit.
 27. An integrated circuit comprising: two power supplyterminals configured to power the integrated circuit, said power supplyterminals including a V_(dd) positive supply terminal and a V_(SS)ground terminal together defining a range of logic levels; logiccomponents in series with a sleep transistor electrically connected toone of said power supply terminals, each of said logic components beinga either a logic gate or a storage cell; a voltage generator configuredto generate a voltage outside said range of logic levels; voltagegenerator control circuitry operably coupled to said voltage generator;circuitry configured to apply said voltage outside the range of logiclevels to said sleep transistor during a power down mode; and a voltageregulator operably coupled to said voltage generator control circuitry.